The present invention relates to a zoom lens system suitable for a projection lens in a liquid-crystal projector that magnifies and projects an image displayed on a liquid-crystal panel onto a screen, and to a projection-type display apparatus incorporating the zoom lens system.
FIG. 24 is a structural diagram showing an optical system of a conventional liquid-crystal projector as a projection-type display apparatus. In FIG. 24, a reference numeral 11 designates a light source such as a metal halide lamp or a xenon lamp, 12 designates an elliptical mirror, 13 designates a reflecting mirror and 14 designates a collimator lens. Further, 2GR, 2G, 70G and 70GB designate dichroic mirrors, 3 and 4 designate reflecting mirrors, 6R, 6G and 6B designate liquid-crystal panels, 21R, 21G and 21B designate condenser lenses, 8 designates a projection lens and SC designates a screen.
A light emitted from the light source 11 is reflected and condensed by the elliptical mirror 12, reflected by the reflecting mirror 13 and impinges on the collimator lens 14, which converts it into parallel light beam 15. The light beam 15 is separated into three primary colors red, green and blue by means of the dichroic mirror 2GR, which reflects green light and red light but transmits blue light, and the dichroic mirror 2G, which reflects green light but transmits red light. Each of the light beams of three primary colors then passes through the respective condenser lenses 21R, 21G and 21B, and impinges on the respective liquid-crystal panels 6R, 6G and 6B. The liquid-crystal panels 6R, 6G and 6B are driven by a drive circuit, which is not shown in the drawings, and each displays a monochromic image of one of three primary colors, red, green and blue, respectively.
The light beams modulated by the images formed on the liquid-crystal panels 6R, 6G and 6B are synthesized into a single light beam by the dichroic mirrors 70G and 70GB, and the synthesized light beam is converted by the projection lens 8 into projection light 9, so that a magnified color image is projected onto the screen SC.
The condenser lenses 21R, 21G and 21B are provided for the purpose of converting the parallel light beam 15 into a convergent light beam, and injecting the synthesized light beam 15 efficiently into the projection lens 8. The projection lens 8 is a zoom lens wherein the focal length is changed, thereby changing the size of the image projected onto the screen SC. Furthermore, focusing of the light projected on the screen SC is also accomplished by moving a portion of the lens group included in the projection lens 8 or the whole projection lens 8 in the direction of the optical axis.
The zoom lens system for a liquid-crystal projector in which primary color images on three liquid-crystal panels are synthesized by two dichroic mirrors is subject to the following requirements (a) to (e):
(a) The zoom lens system must have a back focal length great enough for the color synthesizing dichroic mirrors and reflecting mirrors to be inserted.
(b) The focal length of the zoom lens system at the minimum focal length state must be short enough and the field angle of the projection light must be large enough to form a large picture image at a short projection distance, and the zoom ratio must be large enough.
(c) The resolving power must be high enough for projecting the image from the liquid-crystal panels of high pixel density, and distortion and chromatic aberration must be well controlled.
(d) The number of constituent lens must be small, the overall length of the zoom lens system must be short, and the diameter of the zoom lens system must be small. Also, the structure of the lens barrel must be simple.
(e) In the case that the transmission scattering-type liquid crystals such as polymer dispersed liquid crystal (PDLC) is used, it is desirable that an aperture stop for removing scattered light is provided in the zoom lens system, and the diameter of the aperture stop is variable for adjusting the scattered light removal characteristics.
The zoom lens system including two lens groups suitable for a photographic lens, in which the back focal length is greater than the focal length, is disclosed in Japanese Patent Kokai Publication No. 134452/1977. In this publication, a first lens group has a negative refracting power and a second lens group has a positive refracting power. If it is attempted, however, to make use of a photographic lens in a liquid-crystal projector, such a lens may not be of the ideal structure in terms of field angle, F number, back focal length, number of lens included in the zoom lens system, and so on.
Further, examples of conventional zoom lens systems for use in a liquid-crystal projector, of the type in which light beams from three liquid-crystal panels are synthesized by two dichroic mirrors, are disclosed in Japanese Patent Kokai Publication Nos. 83215/1992 and 119257/1993. The former comprises four lens groups including eleven lens elements and has a lens barrel of complex structure, and because of the large number of constituent lens elements, the manufacturing cost is high. The latter can, despite its two-group composition, provide a back focal length almost double the focal length at the minimum focal length state, but it has no fewer than twelve or thirteen constituent lens elements and the zoom ratio is small, for example, only 1.2.